BROCADE IP PRIMER

Configuring BGPBut how do we use our loopbacks? You need to configure an additional commandwhen you configure your iBGP neighbor:BR-Switch(config)#router bgpBR-Switch(config-bgp-router)#neighbor 4.3.2.1 remote-as 12345BR-Switch(config-bgp-router)#neighbor 4.3.2.1 update-sourceloopback 2The update-source command tells BGP what address your router should becommunicating from when you're speaking to this neighbor. Next, you specifythe interface with the IP address that you want to use as your “from” address.In this case, we chose “loopback 2.” It could just as easily have been “loopback1” or “loopback 4” or whatever. Now, your iBGP neighbor will see the TCPsession as the address of your loopback interface.But I keep specifying iBGP, don't I? Well, it's certainly possible to use a loopbackinterface with eBGP, but it is not very common. Largely, this is due to theexpense of the links. If you are connecting to an Internet Service Provider,each link costs money. More often than not, only one link will be purchased.For redundancy, many engineers purchase additional single-links from differentInternet Service Providers. The point is, if you're dealing with one link, youonly have one path to your switch. The loopback is used because you may havemultiple paths to your switch.Also, let's say you only have one link from your switch to your ISP. When thatlink goes down, you want your session to register as down. That way, BGP willstart sending and receiving over other links. In summary, loopbacks in iBGPare a great idea; loopbacks in eBGP is not usually practical.iBGP - next-hop-selfUsing iBGP with eBGP is great. You've got no metric conversions. You're sharingyour routes inside your own AS. But let's remember one thing. When aroute is advertised through BGP, it is advertised as a path into a new AS. It willgive the AS number and the next hop IP address of a router that will get youthere.Brocade IP Primer 281